Axial movement linear gauges, or gauging heads, that have similar characteristics are from time conventional in the art. Two embodiments are disclosed in U.S. Pat. No. 4,347,492. A first head, illustrated as prior art in FIG. 1 of the U.S. patent, has the typical structure of the aforementioned, so-called "cartridge" heads, including a cylindrical spindle, axially sliding within a casing by means of a guide device consisting of an antifriction bearing and carrying at one end a feeler for touching the workpiece to be checked and at the other end a ferromagnetic core, that translates inside associated windings, as a consequence of axial displacements of the spindle.
The antifriction bearing comprises balls and a cage with holes for seating the balls, that contact both the external surface of the spindle and the internal surface of the casing. The sliding of the cylindrical spindle is enabled by the rolling of the balls on the two surfaces with which it contacts. Consequently, slidings of the spindle cause slidings of the whole cage.
The structures of the cartridge heads disclosed in the U.S. patent are subject to some drawbacks attributable to the great delicacy and care required for the assembly of the guide device.
In fact, for the purposes of a correct performance, the employed antifriction bearings have the need to undergo delicate assembly operations, owing to the fact that, among casing, balls and spindle, there need to be a coupling with a theoretically null clearance, with a very tight tolerance. Furthermore, it is necessary that the various component parts be in well defined reciprocal longitudinal positions, for the sake of preventing inappropriate limitations to the displacing of the cage of the bearing in the course of the slidings between spindle and casing. This means that the component parts have to be manufactured so that one part takes account of the other and assembled with great care, hence implying additional expenses.
The "cartridge" heads with guide devices of this type are subject to further drawbacks, arising from undesired displacements of the cage seating the balls with respect to the previously mentioned accurate longitudinal position. These displacements could be due, for example, to vibrations of the head and the unavoidable clearances that in practice exist. These displacements--more frequently occurring in those applications where the measuring head has a vertically arranged measurement axis--can improperly limit the possibility of the bearing displacing and alter the possible reciprocal displacements between spindle and casing and, as a consequence, vary the measuring range, till jamming the spindle.
Antifriction bearings including balls are used in a number of devices comprising mutually movable elements, such as the probe for coordinate measuring machines that is shown in international patent application WO-A-8301301. The probe has a stylus carrying a tip and angularly movable in any radial direction. Movements of the tip are mechanically transmitted to an external switch through a ball and cone coupling and a shaft, the latter translating with respect to the probe housing by means of an antifriction bearing with balls.
British patent application GB-A-2107410 discloses a recirculating ball-spline assembly to be incorporated in industrial machines such as machine tools and industrial robots and including a sleeve, a pair of cage halves, eight sets of balls and a spline shaft having a square cross-section.